Seed priming is a process for treating plant seeds that enables them to undergo faster and more uniform germination on sowing or planting, with the option of simultaneously treating them with fungicide or other preservatives providing protection during processing or after sowing and allowing their prolonged storage, e.g. in packets displayed at point of sale.
This process allows the seeds to absorb enough water to enable their pre-germinative metabolic processes to begin and then arrests them at that stage. The amount of water absorbed must be carefully controlled as too much would simply allow the seed to germinate and too little would result in the seed ageing. Once the correct amount of water has been absorbed it is then necessary to hold the seed at that water content for a period, typically one to two weeks, before drying it back to the original water content for storage. When subsequently sown the seeds usually germinate more quickly and uniformly than natural unprimed seeds and, where the geographical situation of the point of priming allows, the seeds can be sown directly after priming without drying whereupon they germinate even faster than those which have been primed and dried.
The conventional way of priming seeds has been to immerse them in an aerated solution of an osmotic material, usually polyethylene glycol (PEG). The seed coat is more or less semi-permeable to PEG such that the osmotic potential of the seed tends to equilibrate with that of the solution; the PEG concentration being chosen such that it will not allow seed to absorb enough water to germinate. This works well for many species but the priming of large quantities of seed requires large quantities of PEG solution and this can cause disposal problems, particularly if fungicides have been added. Immersion in liquid also restricts oxygen absorption and some seeds, particularly onions, will only prime satisfactorily if the solution is aerated using air enriched with oxygen.
To avoid the problems of large scale priming with PEG the system known as `drum priming` was devised (see U.S. Pat. No. 5,119,598=GB 2192781). This involves first carrying out tests on a seed lot to determine the optimum level of hydration and then hydrates the bulk of seeds in a drum which is slowly rotated about its horizontal longitudinal axis. Water is added to the drum more slowly than the seeds are able to absorb so that they become hydrated without ever appearing to get wet. The seeds are next incubated in a revolving drum with access to air for a period appropriate to their type before drying.
An alternative method of priming seeds is known as `matrix priming` and does not involve the use of PEG but utilises an absorbent medium such as clay or peat to absorb water and then transfer it to the seeds (see U.S. Pat. No. 4,912,874). The extent of seed hydration is controlled by altering the water content of the medium and the medium/seed ratio. The process is completed by removal of seed from the medium with optional drying. Both drum and matrix priming have been developed to avoid the problems of using PEG when priming large quantities of seed.
Further seed treatments involving control of seed water content are germination and desiccation tolerance induction treatments. Treatment for producing germinated seeds may involve treatment similar to priming as described above with treatment being continued until the radicle emerges. Seed so produced may be further dried back and/or coated as described in U.S. Pat. No. 4,905,411. Separation of the germinated seed from primed seed may be carried out using a seed classifier operating to separate seeds showing signs of radicle emergence from those that do not. Such apparatus and method are exemplified by the disclosure of Kirin Brewery Co Ltd JETRO May 1994 where a video CCD camera is used to observe seed for colour and size of tissue and a compressor device is used to move selected developed seed from undeveloped seed. Treatment for induction of desiccation tolerance in seed with emerged radicles is exemplified by WO 94/05145 wherein the content of the seed is held between 35 and 55% weight such that the emerged radicle does not develop while other metabolic processes continue.